1. Field of the Invention
The present invention relates to a demodulation technique for demodulating differential phase shift keying (DPSK) modulated received signals.
2. Description of the Related Art
Recently, various standards or types of wireless communication systems have been proposed in order to facilitate data reception/transmission between a number of electronic devices. For example, a Bluetooth standard compliant communication system DPSK-modulates carrier waves using digital modulation data.    Patent Document: Japanese Patent Application Laid-Open No. 2002-290294
In DPSK modulation, a phase shift amount between neighboring symbols may correspond to the amount of codes to be demodulated. Phase data representing a phase of the received signal are oversampled by an integer multiple, for example, 8 or 16 times of a symbol rate. When symbols are extracted from the oversampled phase data, according to a conventional demodulation circuit, the phase shift amounts between a piece of the phase data employed as a previous symbol and each piece of the phase data being currently sampled are computed, and a phase difference data most approximate to a predetermined expectation value is employed as a current symbol.
This method may be applicable on the assumption that the phase of the previous symbol is accurate. However, under a practical communication environment, the symbol is difficult to be consistent with an ideal constellation. When the previous symbol is not consistent with the ideal constellation, it is difficult to accurately evaluate the phase shift amount, and this may influence selection of a current symbol, so that receiving sensitivity may be degraded. That is, this method has an algorithm significantly affected by accuracy of the previous symbol.